1. Field of the Invention
The present invention relates to an automatic exposure device for use in an image: forming device such as an electronic copying machine.
2. Description of the Prior Art
A conventional automatic exposure device for use in a copying machine is arranged so as to detect the amount of light reflected from an original document to be copied by a light sensor and to control the power of a light source for illuminating the original document based on the light signal obtained by the light sensor. In case where detection indicates that the original document is dark, power supplied to the light source is increased, alternatively, when detection indicates that the original document is bright, the power supplied to the light source is decreased, in order to assure an optimum exposure to the original document corresponding to the density of the picture of the original document to prevent a fogging of the copied picture. In the automatic exposure control device mentioned above, although the power to the light source is increased if it is detected that the original document is dark, the light source emits excessive light without an upper power limit. The problem that results with the aforementioned process is that if an original with slight lines near very dark characters is copied, the lines and/or characters disappear in the copied picture. To solve this problem, therefore, an upper limit of the supplied power level for the light source is set. In the conventional automatic exposure control devices, the upper limit of the power level supplied to the light source is fixed.
However, there may occur a case that the starting up time of the light value emitted from the light source differs greatly depending on the kinds of the light source and the temperature of the atmosphere of the light source. For example, when a fluorescent lamp is used as the light source, the starting up time of the light from the lamp is delayed greatly if the atmosphere temperature is low compared with the lamp driven in a normal room temperature and there may occur such a case that the amount of the light reflected from the original document can not reach a sufficient light level even if a full power is supplied to the lamp. If the power to the light source is limited to the upper limiting level under the condition mentioned above, the amount of the light reflected from the original document becomes insufficient, resulting in making a copy that is foggy.